Efficient utilization of lumber production requires that the material be graded according to its intended use. In this way, an effective and economic match can be made between the lumber needs of end-users and the lumber supplier of the product. Many factors control the suitability of lumber for any particular purpose. They include the degree of straightness, the amount of any wane, and the presence and size of knots, splits, shakes, etc. These and other factors are currently assessed by trained personnel using established visual grade rules.
Several engineering properties, including tensile strength, bending strength, and bending stiffness are of great importance when designing wood structures and factor greatly in the suitability of a particular piece of processed lumber for a specific application. For example, lumber having a high bending stiffness or Modulus of Elasticity (MOE) is worth more than lumber with low MOE, since lumber with a higher MOE can be used in such applications as floor joists or roof trusses, which span over a longer distance, or provide a “stiffer” floor or roof over the same span is needed or required.
In the visual system of grading, these aforementioned properties have been established from destructive tests on extensive samples of each visual grade, species, and size of lumber. Mechanical grading on the other hand, indirectly measures these properties on each piece, and is independent of species and size of the material. The process of visual grading includes a wide range of wood strength and bending stiffness. Thus, a sample of material of a given visual grade contains pieces whose strengths and bending stiffness vary over very wide ranges. For example, the strength of the strongest piece in a batch of a given visual grade is typically 5–10 times that of the weakest piece. Thus, for safe design, a near minimum strength of the population has to be assumed. This is clearly very wasteful of the majority of superior pieces which are being used at well below their actual capacities. In addition, these properties vary according to the species and size of the material.
Such waste can be reduced by developing and using techniques which better identify the superior pieces and reliably distinguish them from the inferior pieces. One such non-destructive technique has been developed to overcome the deficiencies of visual inspection, yet still provide a determination of mechanical properties in the wood product. This technique employs X-ray imaging to measure the density of the wood product, and to image defects such as knots. From the measurement of density, the bending stiffness of the wood produce can be inferred. In addition, by taking into consideration the size and location of the defects, the strength of the wood product can be estimated.
The X-ray imaging technique, however, has the drawback that density determined by X-rays is not completely indicative of strength, nor can bending stiffness be reasonably inferred from only the density. In particular, there are growing conditions in which the density of the wood is “normal”, and yet the boards have low bending stiffness. Biological deterioration can degrade the mechanical properties of wood, yet does not change the density of wood. One such condition is “compression wood”, which is caused by trees growing on steep hills, or in regions of constant prevailing winds in a specific direction. In these cases, the wood has different structural properties on the uphill (upwind) side and the downhill (downwind) side of the tree. Another condition is when wood products are manufactured from young plantations. This material produces another condition in which the density within the tree may appear “normal”, yet the bending stiffness of the wood product varies tremendously. Such plantation wood typically has similar densities as old growth trees of the same dimensions, but typically includes a higher percentage of wood that exhibits juvenile characteristics, which includes a greater microfibril angle and varying quantities of chemicals. Wood products having higher concentrations of juvenile characteristics are prone to having extreme variations in bending stiffness.
Therefore, there is a need in the wood products industry for improved systems and methods that can predict the bending stiffness of wood products.